I'm a geochemist. In the past ten years I've fixed mass spectrometers, blasted sapphires with a laser beam, explored for uranium in a nature reserve, and measured growth patterns in fish ears, and helped design the next generation of the world's most advanced ion probe. My main interest is in-situ mass spectrometry, but I have a soft spot in my heart for thermodynamics, drillers, and cosmochemistry.

Thursday, July 03, 2008

This is a brief technical note for any ICPMS users out there who are trying to analyse elements with carrier gas interferences. Although I am not in the private sector, I still interact socially with academic researchers from time to time, and a post doc recently approached me with a analytical problem that may be related.In laser ICPMS analysis, it is commonly assumed that any background that exists in the carrier gas prior to sample acquisition will remain constant when the laser comes on and ablation introduces sample into the system. However, as a result of this recent query I dug up some 2 year old data where I investigated this effect, and I noticed that for gas interferences with high first ionization potentials, sample introduction can reduce the background. Thus if you subtract the unreduced background from the sample signal, you will oversubtract, and underestimate your abundance.The following table shows a) the mean background counts for one Xe and two Hg isotopes (which are both contaminants in the Ar gas); b) The mean counts during signal acquisition, and c) the ratio of signal/background.

Note that when ablating the doped Na-Ca-Al-Si NIST 612 glass, the Xe signal is reduced to 93% of the background, while the Hg is reduced to 98%. Interestingly, this effect seems to be matrix-specific; ablation of rutile only reduced the Xe to 98% while leaving Hg unchanged.I have no idea if this sort of thing has been published- its easier to mine the data then slog through a library which I can’t access. And since I’m not an academic, I don’t particularly care. But chances are this sort of effect is machine specific, so anyone trying to measure isotopes such as 130Te, 82Se, or 204Pb would be wise to check their own instruments before trying to quantify unknowns.

Disclaimer:

All opinions, measurements, figures, and facts on this page are the personal opinions of Charles W. Magee, Jr, and do not represent the views of any of his employers: past, present, present-but-about-to-be-past, or future. None of the content herein has been subject to peer review, and should be treated with caution or derision. Any passing mention of OSHA code violations, criminal activities, unethical or unscientific behavior, or the clandestine Australian nuclear weapons program are fictions created to make rhetorical points, and do not represent the reality of my, or anyone else's, workplace. Do not attempt any scientific protocols described herein at home, with the exception of the chocolate chip cookie recipe. Do not apply the products of that protocol to individuals with heart disease, diabetes, high blood pressure or cholesterol, egg, wheat, dairy, or chocolate allergies. Do not view this blog continuously for more than 45 minutes without stretching and taking other precautions to prevent computer-related chronic injury.
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